With implanted devices it may be necessary to transmit information to the implant over a comparatively short distance during an extended period of time.
An example for such an application is the transmission of speech information to a fully implanted cochlear implant 101 from an external device 102, such as a microphone 103 and/or a processor 104 placed behind the ear or in the ear canal, as shown in FIG. 1. Since the transmitter 105, as well as the receiver 106, are powered from small batteries 107 and 108 contained in the external device 102 and in the implant 101, respectively, both the power consumption of the transmitter 105 as well as of the receiver 106 become limiting factors.
Another application could be the transfer of information between bilateral ear level microphones and/or processors used with hearing aid or cochlear implant applications. In these cases a comparison or a common processing of left and right speech signals may be necessary either for beamforming or for coordinated processing schemes in order not to distort direction information.
Referring to FIG. 1, a cochlear implant transmission system typically includes an Radio Frequency (RF) transmitter 105 that drives an external coil 109 with a modulated RF signal. This signal is picked up by a receiver's 106 implanted coil 110, which may be located at only a few mm or cm from the transmitter coil 109, and further processed by the receiver 106. With conventional narrow band RF transmission schemes using well known modulation methods for transmission of speech signals of considerable dynamic range (e.g. 70-90 dB) either as an analog signal or a coded signal (e.g. PCM or encoded in a ΣΔ-modulator-data stream, typical band width 1 . . . 2 MHz), the power consumed either by the transmitter 105 or by the receiver 106 (e.g. when using a very low power transmitter delivering a very faint signal making a large RF amplification necessary), or both, may turn out to be prohibitive. Note that the overwhelming percentage of the total power consumption results from the RF components at the transmitting and/or receiving end. The power consumption of processing in the baseband is negligible due to the low speed power product of modern CMOS technology and the comparative low frequencies of the baseband.
Very low power receivers may utilize, for example, diode rectifiers. However, the threshold voltage of the diode rectifier may be too large, even when using backward diodes for demodulation. Another very low power receiver is the superregenerative receiver, which does not have sufficient bandwidth for transmission of coded speech signals. Examples of still other receivers include superheterodyne or the homodyne receivers, or a straight amplifier chain preceding a demodulator. However, in each of these cases the power consumption of the RF amplification is non-negligible. Depending on the transmitter power selected, the relative proportion of transmitter power to receiver power may be adapted to the respective battery capacity available. For example, a strong transmitted signal may require small or even no amplification at the receiver. However, total power consumption may be too large in any event.